Anti-Ro/SSA autoantibodies are a common feature of the autoimmunity of systemic lupus erythematosus, Sjogren's syndrome, subacute cutaneous lupus erythematosus, and neonatal lupus syndrome. Numerous clinical immunogenetic and laboratory associations have been found with this autoantibody. Moreover, there is strong evidence that anti-Ro/SSA antibodies are concentrated in the skin, kidney, and parotid glands of some patients and, hence, are capable of being pathogenic. This project of the Program will first characterize the sequential autoepitopes of the 50kD Ro/SSA protein. The preliminary data demonstrate that the fine specificity varies between patients. Indeed, in some patients the apparently minor differences between bovine and human Ro/SSA are sufficient to destroy antigenicity and have led to the hypothesis that the human Ro/SSA antigen is an autoimmunogen. The human cDNA protein sequence is available from our previous work. In these experiments, the cDNA for the 60 kD bovine Ro/SSA will be identified, cloned and sequenced. The bovine and human sequence will be used to investigate the antigenic differences between the 60 kD bovine and human Ro/SSA proteins. Overlapping peptides have been synthesized to the carboxy terminal 13 kD of human Ro/SSA and one of its epitopes identified. Once sequential epitopes are preliminarily identified, they will be characterized for optimal size and amino acid sequence. Indeed, structures may be revealed that are more reactive than those in human Ro/SSA. Once this has been done, fine specificity will be tested in existing collections of sera from patients with systemic lupus erythematosus to assess antibodies to particular epitopes of Ro/SSA with the known clinical, laboratory and immunogenetic relationships to anti- Ro/SSA. In later experiments, carefully selected bovine-human hybrid molecules of Ro/SSA will be prepared and their antigenicity assessed. The reactivity of these molecules will lead toward an understanding of the conformational epitopes. This work has the potential not only to reveal important structural features of an important rheumatic disease autoantigen but also to define its optimal antigenic structure as well as the different clinical, immunogenetic and laboratory findings associated with autoanti- Ro/SSA at the level of the fine specificity of this autoantibody.